One challenge to the development of THz semiconductor devices, particularly transistors, is THz measurement and characterization. As it stands today, vector network analysis (VNA) with standard (metal-to-metal) dc-coupled contact probing is commercially available up to 500 GHz (Oleson Microwave, http://www.omlinc.com/products/vna-extension-modules/wr-022-325-500-ghz.html) using coaxial probes (GGB Industries, Inc.; PicoProbe Model 325; http://www.ggb.com/325.html), with promise of extending to 750 GHz using silicon-micromachined probes (Dominion MicroProbes, Inc; Model DMPI 1.5V01MPR; http://dmprobes.com/Products %20DMPI.html). However, this technology is very expensive and fragile, with little hope of working beyond 1.0 THz in the foreseeable future, in large part because it requires application of vector network analyzers and custom front-end frequency-extension modules and down-conversion units. The fragility stems from the small size of the ground-signal-ground (GSG) probes and their need to make intimate metal contact. In technical terms, the existing technology couples the conduction current term, J, in Maxwell's equations (specifically, Maxwell's generalization of Amperes Law (∇×H=J+∂D/∂t)). In circuit terminology, this is “dc coupling.”
Photomixing entails the use of two fiber-coupled, frequency-offset diode lasers (usually distributed feedback lasers) to generate or receive a single difference-frequency tone in a photomixer—an ultrafast photoconductive gap easily embedded in a terahertz (THz) antenna or planar transmission line. The tone is “pure” in the sense that there are no harmonics and no intermodulation products of any sort. An important development over the past 10 years is fully-coherent photomixing. There is a transmit (Tx) photomixer and a receive (Rx) photomixer, both driven by the same pair of diode lasers so that the difference frequency tone at each photomixer is mutually coherent. Difference-frequency sweeping occurs by temperature tuning of one or both lasers. Mixing of the Tx-radiated tone with the Rx-generated tone produces a dc component that is easily read out to a transimpedance amplifier. This is the same “homodyne” conversion common in RF and photonic transceivers. To avoid dc-drift and 1/f-noise issues, the Tx photomixer is easily amplitude modulation (AM) or frequency modulation (FM) modulated, and the phase-preserving Rx baseband is raised to the modulation frequency (quasi-homodyne).